~ Casting light on great ideas

Dying for sex

Unfortunately, I’ve been too busy to attend to Inspiring Science this week. Rather than putting out a rushed post, I decided to republish this piece which I originally wrote for Accumulating Glitches last year. I hope you like it!

Some spiders get eaten by their mates, and male salmon famously fight to the death for access to females, but we generally don’t think of reproduction being quite as risky for mammals. We may prance and pose or jockey for attention, and mating might even be quite painful, but it’s usually not lethal. Among mammals, “live to mate another day” seems to be the guiding principle. Exceptions to this rule are found in the dasyurids and didelphids, groups of small carnivorous marsupial species living in Australia and South America, respectively. “These species experience extreme sexual behaviour,” said Dr. Diana Fisher of the University of Queensland. Males and females mate with multiple partners and matings can go on for many hours. Afterwards, the males all die.

Dr. Fisher decided to investigate this remarkable behaviour. Together with colleagues from the University of Sydney and the University of Tasmania, she looked at over 50 dasyurid and didelphid species with varying rates of male survival after mating, recording various data like the proportion of males that die after mating, the size of their scrotum and the length of the breeding season. One theory suggests that male die-off might be an altruistic act to avoid competing with their young for food, so Fisher’s team also measured the abundance of their arthropod prey throughout the breeding period. They found that females do time their mating period to ensure peak prey availability during weaning, but the males aren’t sacrificing themselves for their young. Instead, the mass male suicide after mating seems to result from their intense competition. “They use up all possible energy and body tissues on competitive mating, which causes synchronized death after mating in males,” said Dr. Fisher.

The males are forced to such lengths because females will only mate during a brief time window in order to make sure there’s enough food available when they’re weaning the young. Since both sexes are promiscuous, males have to compete with their sperm rather than monopolizing access to the females. They put everything they can into mating, producing over-sized testes and copulating for nearly ten hours at a time. These long bouts limit the female’s access to other males, but after that it’s up to the sperm to slug it out with competitors. “Males compete not by fighting, but by mating themselves to death because their sperm is in competition with the sperm of many other males,” according to Dr. Fisher. The immense pressure to produce better sperm takes its toll on the males, leading to immune failure and eventually death. “Females not only benefit by weaning their young when there is most food,” said Dr. Fisher, “but also by promoting this extreme sperm competition, because the highest quality males father their young.”

This is a great story about sexual selection, about females manipulating males into literally dying for sex, but it’s more than just that. What really got me excited about this story is why the females force the males to compete so intensely — because they want to wean their young when there’s plenty of prey around. In other words, the population dynamics of the prey is driving sexual selection in these marsupial predators. The seasonal changes in the arthropod population ultimately lead to suicidal reproduction in the males of these small mammals. It’s a beautiful story at the intersection of ecology, evolution, and behaviour, a wonderful reminder of the intertwined complexity that is biology.

ReferenceFisher DO, Dickman CR, Jones ME, & Blomberg SP (2013). Sperm competition drives the evolution of suicidal reproduction in mammals. Proceedings of the National Academy of Sciences of the United States of America, 110 (44), 17910-4 PMID: 24101455